Grinding fluids used in grinding can be roughly classified into water-soluble grinding fluids, which have water as a base, and water-insoluble grinding fluids, which have oil as a base. Water-soluble grinding fluids have excellent cooling properties and environmental friendliness, whereas water-insoluble grinding fluids have excellent processing performance and swarf washability. In general, in grinding of small machine parts, a water-insoluble grinding fluid is used because of the importance of productivity and quality, whereas in grinding of other parts, a water-soluble grinding fluid is used because of the importance of cooling.
In the cutting of hardened steel, which has been replacing grinding in recent years, swarf becomes hot and poses the risk of catching fire; therefore, the use of a water-soluble fluid that is non-flammable is required.
Among various types of grinding, superfinishing grinding (hereinafter simply referred to as “superfinishing”) is a processing method for obtaining a smooth and high-precision processed surface in a short time. This method is often used in a final finishing step for machine parts, such as bearing parts.
In superfinishing, superfinishing grindstones designed specifically for superfinishing are used. A superfinishing grindstone generally has a bar-like or cylindrical shape. While vibrating, oscillating, or rotating the grindstone, the grindstone is pressed at a constant pressure against a workpiece that is rotating at a high speed, thus processing the workpiece.
As abrasive particles of a superfinishing grindstone, CBN (cubic boron nitride), WA (white alumina), GC (green silicon carbide), diamond, etc., can be used. The particle size of a superfinishing grindstone is generally about #400 to #8000 and has a particle size smaller than that of grindstones used in general grinding. Since a superfinishing grindstone comprises such fine grinding particles, swarf produced in superfinishing is much finer than swarf produced in general grinding. Furthermore, since superfinishing is performed with a superfinishing grindstone under constant pressure being in contact with the surface of the workpiece, liquid exhaustion on the processing surface is likely to occur, which results in sudden stagnation of the processing due to local clogging of the grindstone. In view of such processing characteristics, water-insoluble grinding fluids that have excellent liquid permeability to the processing surface and excellent diffusibility (washability) of fine swarf in liquid have been generally used.
For example, inner and outer races of a bearing can be manufactured by using a manufacturing process as illustrated in FIG. 1. In the grinding steps before superfinishing, a water-soluble grinding fluid is used as grinding fluid, whereas only in superfinishing, which is the final finishing step, a water-insoluble grinding fluid is inevitably used for the reasons described above.
The water-insoluble grinding fluid for superfinishing, however, is generally classified into the Fourth Group, Third Class Petroleum, of Hazardous Materials, and is designated as a flammable hazardous material under the Fire Defense Law of Japan. A grinding fluid for superfinishing that is supplied to the processing point is stirred and misted by the motion of the workpiece and tool. Accordingly, there has been strong demand for improving the work environment for the superfinishing step. Although a water-soluble grinding fluid used in general grinding may also be used for superfinishing, conventional water-soluble grinding fluids for general grinding have unsatisfactory liquid performance; therefore, using such a conventional water-soluble grinding fluid for superfinishing causes the problem of grindstone clogging, thus leading to failure in processing.
Proposed as water-soluble grinding fluid for solving this problem is a water-soluble grinding fluid that contains not inorganic salts but an alkaline component that inhibits polynuclear complexation by forming a coating layer on ferrous hydroxide formed on the surface of a workpiece to be processed and thereby inhibiting the ferrous hydroxide from converting to a polynuclear complex (PTL 1). The grinding fluid disclosed in PTL 1 inhibits the reaction of converting ferrous hydroxide to ferric hydroxide or the reaction of converting ferric hydroxide to a polynuclear complex by the function of the component inhibiting polynuclear complexation, thus inhibiting adhesion of swarf to the grindstone and preventing clogging; however, the effect of inhibiting grindstone clogging was insufficient with this method.